Hypothesis: The influence of hydrophobic nanocellulose on phase separation behavior of off-critical PS/PVME (polystyrene/polyvinyl methyl ether) blends was studied. While the effect of spherical nanoparticles (NPs) on the phase behavior of polymer blends has been previously explored, the impact of rod-like NPs on the phase behavior has not been well studied. Compared to nanospheres, nanorods are associated with much lower critical percolation concentration, due to the high aspect ratio of nanorods.Methods: For this purpose, neat PS/PVME blends with compositions of 40/60 and 10/90 and in the presence of 2% nanocellulose were investigated. The temperature sweep of storage modulus, from the one-phase region temperature to those higher than the two-phase region temperature, was used to investigate the effect of nanoparticles on phase separation temperature. Phase-contrast optical microscopy (OM) was employed to investigate the morphological evolution of PS/PVME blends during the phase separation. TEM images indicated the localization of hydrophobic nanocellulose in the PS-rich phase which was consistent with the prediction of thermodynamic wetting parameter.Findings: Viscoelastic phase separation (VPS) controlled the phase behavior of PS/PVME 10/90 blend which in the presence of nanoparticles increased the stability of the PS-rich network structure even at high temperatures. The PS/PVME 40/60 blend was phase separated under the nucleation and growth mechanism (NG), and there was a wide distribution of droplets size in the late stage of phase separation. With increasing the quench depth, the dynamic asymmetry increased, leading to transition of the phase separation mechanism from NG to VPS. The addition of nanoparticles enhanced the dynamic asymmetry which induced the VPS at lower temperatures.